There's nothing odd about it, and it's already been replicated (mine is numbered almost 300 numbers higher). It is a fully functional belt loader, of the style from late WWII. They work fine, and load two cartridges per single push of the lever.
I believe the one on Gunbroker to be a post war reproduction. I think I gave $120 (including postage) for mine about 4 years ago. Works great, but $700 would be high even if it was genuine wartime. "German marked" doesn't mean anything. Stamps are sold on ebay for nearly any mark you want, for about $35 each. Mine was faked with an actual waffenamt, rather than the bird shown in the Gunbroker picture, but it's the same manufacturer. It was new/never used when I bought it.
It loads a belt at about the same speed as a crank loader, but you don't need to clamp it on a table. You can put it on a floor and use your foot.